Two-way carrier wave telephone system



Ap 27, 1948. M, L, ALMQUls-r 2,440,239

TWO WAY CARRIER WAVE TELEPHONE SYSTEM Filed Dec.' e, 1945 3 sheets-sheet 1 /N VEA/TOR M LALMQU/S 7 BV ATTORNEY M. l... ALMQuls'r 2,440,239

Filed Dec. s, 1945 s sheets-sheei'z TWO WAY CARRIER WAVE TELEPHONE SYSTEM l April 27, 1948.

/A/VE/VTOR M L. A/ MQu/s @y l We zi ATTORNEY April 27; 1948. M.1...A1 MQu|s'rl l 2,440,239,

TWO WAY CARRIER WAVE TELEPHONE SYSTEM 7' 0 POWER SUPPLY/6 A TTORNE V Patented Apr. 27, 1948 @UN-irren STATES PATENT OFFICE Y. TWO WAY TELEPHONE i `'SYSTEM Milton L. Alniquist, MaplewomL'N. J., assigner .toBell :Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 6, 1945, Serial Na-33,205

9` Claims. (Cl. 179-15) rIhis invention relates to an electric Wave transmission vsystem. and 4particularly to an electric communication system employing high frequency carrier waves for transmitting the communication signals.

Thefinvention is specically applicable to a communication system Aemploying anumber of high frequeneycarrier channels to provide twoway telephone service to telephone subscribers `ovena.common transmission'medium. In such a` system,v the common medium may bea transmission line, for example, one Vused primarily for supplying low frequency,V high 4voltage power to .a number. of powe: consumers in apower distribution network, .or .oneor more radio transmission links.. .Each carrier channel may be .utilized for .providing vtwo-Way communication between eachrtvio telephonesubscribers on a party line basis over thezline. or .other common transmission. medium through an individual carrier terminal havinganassociated central switchinglofice common to the several channels. The central. ofce .associated with the carrier terminalsmay providemeans for 'connecting the carnier telephone .subscriber stations associated with each carrier channel. the common medium. in communication relationwtheach otherv or with .other telephone subscriber stations on .other carrier channels .onthe same medium or. on outsid systems.

In. SuchA prior art systems, it is. known to. utilize twodiifercnt carrierfrequencies for eachlcarrier channel .for transmission between the carrier terminal equipment and the-subscribers. equipment, one for each direction. of transmission. subscribers equipment is normally arranged @transmit on one carrier; frequency f2 and to receive. cna second ycarrier frequency fi and the carrier,,terminalequipment is arranged to trans- ;itson carrier irequencyfi and receive on carrier freuuency ,fz-orji. To make a reverting or party iine calL'that is a call between subscribers onthesamecarrier channel, the transmitting and-receiving frequencies of either the calling or called'stationl are reversed. In thisY case it is necessary to prevent transmission of the carrier frequehcyi vfram the carrier terminal .during thecallsince the presence of vthis Afrequency `would cause beats with the subscribers transmitter using this same frequency and thus produce distortion in the transmitted communication signals. In-acldit-ic.n,y it is necessary to proiideagbusy'indication at thefcentral cnice.

anpobject .ofthe invention isgto improve this generaltyne of system fromthe .standpoint of circuit simplicity, economy of equipment, reduction in frequency selectivity requirements and operating procedures.

Another object is to provide two-way communication between any two of a number of subscribers associated with each carrier channel of a multichannel carrier communication system on a party line basis, as well as to provide two-way Acommunication between any subscriber on each carrier channel of such a system and another subscriber on another carrier channel of the same system or on any other system or line to which this carrier system may be connected.

' in accordance with the invention, a multichannel carrier communication system of the general type described above is modied so that all communication connections, both between the subscribers associated with eachcarrier channel'on thelineor other common transmission medium and other subscribers outside that channel or carrier system, and between subscribers on the samey carrier channel of the carrier system, are made through the centralofce in the same manner. This is accomplished by giving each subscriber on each carrier channel access to a second carrier channel onthe common transmission medium to which he can Vconnectby operation of a key provided @n his subscribers communication set, Each subscriber of a group assigned to one carrier channel on the common transmission mediumnormallyutilizes one carrier frequency for receiving from the carrier terminal for that channel and a second carrier frequency for transmitting to'that carrier terminal. Provision is made inthe subscribers communication set whereby the transmitting and receiving carrier frequenciesof the set can be shifted to other diierent frequencies'which are respectively the normal transmitting and receiving carrier frequencies utilized by the subscriber sets associated with another carrier channel on the common transmission medium.

In making a call to a subscriber outside the carrier system or to a subscriber on another carner. channel of the same system, the subscriber associated with one carrier channel `on the common transmission medium merely has to lift his receiver. or telephone handset off the switchhook, in which casehis set is conditionedfor transmitting on one carrier frequency and receiving on a second carrier frequency. The first carrier frequency is transmitted over .a common medium tei-.he carrier terminal for that channel where it is utilized to call in the central .oce operator, forexampleby closing .a normally open transmission loop to the central oiiice to operate a suitable alarm device, and to cause the energization of the normally disabled transmitter at the carrier terminal so that it sends out the second carrier frequency over the common transmission medium to operate a busy signal at each of the subsciiber stations on the same carrier channel.

To make a party line call, the calling subscriber on any channel normally transmitting on one carrier frequency and receiving on a second carrier frequency, when the unoperated condition of his busy signal indicates that the channel is idle, presses a non-locking key which causes his set to be conditioned for transmitting on a third carrier frequency and receiving on a fourth carrier frequency normally assigned to a second carrier channel on the same common transmission medium. If this second carrier channel is idle, no busy signal will be received at the call-v ing station. The subscriber then lifts his receiver or handset on? the hook, thereby locking his set on the alternate frequency assignment and sending the third carrier frequency out over the common line or other medium to operate a busy signal at each subscribers station on the alternate carrier channel. The received third carrier frequency at the carrier terminal for the alternate channel will control switching arrangements operating to call in the operator at the central ofiice, and to energize a transmitter at that carrier terminal to send out the fourth carrier frequency over the common medium. The central oihce operator then completes the call to the called subscriber on the subscribers normal channel through the central onice switchboard in the usual manner, and the two subscribers converse with each other through the central oiiice. When the call is completed, the calling subscriber hangs up his receiver and his set automatically reverts to the frequencies of the carrier channel to which he is normally assigned.

A more complete understanding of the invention will be obtained from the following detailed description when read in connection with the accompanying drawings in which:

Figs. 1 and 2, when placed side by side with Fig. ll at the left, show in block schematic form a power line carrier telephone system embodying the invention; and

Fig. 3 shows schematically one type of variable frequency carrier oscillator and iband filter switching arrangement and the control circuits therefor, which may be used at each of the subsubscribers stations associated with one carrier channel in the system of Figs. 1 and 2; and

Fig. 3A shows schematically a modification of a portion of the arrangement of Fig. 3 to make the arrangement suitable for use at each subscriber station associated with a second carrier channel in the system of Figs. 1 and 2.

The power line carrier telephone system of Figs. 1 and 2 includes a transmission line I primarily employed for transmission and distribution of low frequency, high voltage, electric power and adapted for simultaneous use for high frequency carrier wave telephony. The line i is shown as a single phase line having one wire grounded but the carrier telephone system to be described may be applied to any type of power or other transmission line. A plurality of two-way carrier telephone channels having different frequency assignments are applied to a given section of the power line I., of which only two channels, designated channels I and 2, have been illustrated in order to explain the invention. Each channel may form a party line with six to' eight, or possibly more, subscribers. As shown, channel I comprises a plurality of subscriber two-way carrier` telephone stations designated A, B and an individual portion CT1 of a two-way carrier terminal CT, and channel '2 a plurality of subscribers two-way carrier telephone stations X, Y and other individual portion GT2 of the two-way carrier terminal CT. Each of the subscribers stations A, B and X, Y and the carrier terminal CT are bridged across the power line I by suitable coupler arrangements 2, which may comprise condensers or filter arrangements (not shown), adapted for permitting the transmission of the high frequency carrier signals to or from the line and for substantially preventing transmission of the Alow frequency, high voltage power from the line into the subscribers carrier telephone and the carrier terminal equipment.

Each of the portions CT1 and CTz of the carrier terminal CT are adapted for connection to the common central switching ofce, CO, through individual line loop connections to be described later.

The subscribers stations A, B in channel I and the subscribers stations X, Y in channel 2 are identical except for thev changes in the carrier oscillators and band lters and the frequency changing control mechanisms therefor required because of the dierent transmitting and receiving frequencies assigned to the two channels for the normal and switched operating conditions, to be described.

Each of the subscribers stations A, B, and X, Y as shown, comprises a transmitting circuit 3 and a receiving circuit 4. The transmitting circuit 3 includes in order, reading from right to left, -a telephone transmitter 5 (which may be part of a standard hand telephone set as illustrated in`Fig. 3), an audio amplifier Ii, a modulator 'I with its associated carrier oscillator 3, a high frequency transmitting amplifier 9 and the transmitting band filters Iii and I0 with an associated change-over relay circuit R1,

f feeding into the line I through the coupler device 2. The receiving circuit 4 includes in order, reading from left to right, the receiving band lters II and II" with an associated change-over relay circuit R2, fed from the line I through the coupler device 2, the high frequency receiving amplier I2, the ldemodulator I3 with an associated carrier oscillator I4, and the telephone receiver I5 (which may be combined in a standard hand telephone set with the telephone transmitter 5, as shown in Fig. 3.)

The modulator 'I and the demodulator I3 may be of any of the well-known types including one or more electron discharge tubes or other variable resistance devices, respectively adapted to combine high frequency carrierl oscillations supplied to its carrier input terminals with the audio frequency signals supplied to its signal input terminals so as to produce signal side-band components as well as the unmodulated carrier component in its output, and to combine high frequency carrier oscillations supplied to its carrier input terminals with the unmodulated or modulated carrier signals supplied to its signal input terminals to produce the rectified carrier or the modulating audio signals in its output.

The carrier oscillator 8 associated with the modulator 'I and the carrier oscillator I4 associat-ed with the demodulator I3, when energized by the common power supply I6 by the closing of a switch, which may be the switch contacts IIa.

lower contacts of relay 33 so as to respectively connect the based filters I and I I to the output of the high frequency amplifier S in the transmitting circuit 3 and the input of the high frequency amplifier I2 in the receiving circuit 4, to effectively disconnect the band lters I0 and II from these respective circuits and to switch the resistance terminations from filters III' and II to the filters Iii and I I, respectively.

As illustrated in Fig. 3A, at each of the subscribers stations X, Y associated with channel 2 in the system of Figs. 1 and 2, when the relay 33 is in the unoperated condition, the switching contacts of the change-over relay circuits R1 and R2 at each station may be arranged so that the band filters I0 and II', adapted to respectively selectively transmit the .unmodula-ted frequencies i12 and f2, which are the normal transmitting and receiving carrier frequencies assigned to stations associated with carrier channel 2, are connected to the output of the high frequency amplifier 9 in the transmitting circuit 3 and to the input of the high frequency amplier I2 in the receiving circuit '4, and the band filters I and II, respectively, adapted to selectively transmit the unmodulated and modulated carriers of the frequencies fn and f1, which are the normal transmitting and receiving carrier frequencies of the subscribers stations associated with carrier channel I, are eifectively disconnected from these circuits and terminated with the resistors RT and Ra, respectively. When relay 33 at the station is operated in response to operation of the key 32 by the local subscriber, the change-over relay circuits R1 and R2 so modied are operatively energized in series'from the battery 34' through the lower made contacts of relay 33 so as to respectively connect the band filters ID and I I to the output of the high frequency amplifier 9 in transmitting circuit 3 and the input of the high frequency amplifierl I2 in receiving circuit 4, to effectively disconnect the band lters I3 and I I from these respective circuits and switch the resistance terminations Rr and RR from band nlters I 0 and II to band filters I3' and II', respectively.

The portion CT1 of the carrier terminal CT, associated with carrier channel I on power line I in the system of Figs. l and 2, comprises a transmitting circuit 35 and a receiving circuit 36. The output of the transmitting circuit 35 and the input of the receiving circuit 35 are connected in parallel with each other and through the coupler device 2 to the line I. The input of the transmitting circuit 35 and the output of the receiving circuit 3s are connected by means of the hybrid coil 3l and associated line balancing network 3S to the two-wire voice frequency line 39 leading to the common central telephone switching office CO for the subscribers stations associated with both carrier channels I and 2 on the power line I. The central oliice CO, which may be either of the manual or automatic type, is utilized also for connecting' the telephone subscribers associated with each carrier channel of power line I t0 any of the telephone subscribers stations outside the carrier system in the associated telephone network. The central office CO is normally disconnected from the carrier terminal portion CTi due to the normal break in the line 39 provided through the normally open, parallel-connected left-hand switching contacts of the relays 4| and 42 to be referred to later. The operation of either one of these two relays to close its lefthand switching contacts will complete the connection of the line 33 to the central ofiice CO.

The transmitting circuit 35 of carrier terminal portion CT1 includes in order, between the hybrid coil 31 and the coupler device 2, reading from left to right, an audio amplier 43, a modulator 44, similar to the modulator 'I in each carrier subscribers station, with an associated carrier Y oscillator for supplying thereto carrier oscillations of the high frequency f1, the high frequency transmitting amplifier 46 and the transmitting directional band filter 41, adapted to transmit one or both signal side-band products as well as the unmodulated carrier component of the frequency f1 received from the output of the modulator 44. The receiving circuit 36 includes in order, between the coupling device and the hybrid coil 3i, reading from right to left, the directional band lter 46 adapted for selectively transmitting the speech-modulated and unmodulated carrier waves of the frequency fu received from the line I, the high frequency ampliiier 49 and the demodulator 53 for demodulating the applied high frequency carrier signals of the frequency f 11 to voice frequencies, and for rectifying the applied unmodulated carrier of that frequency.

'Ihe audio amplifier 43, the high frequency transmiitting amplifier 46, and the carrier oscillator 45 associated with the transmitting circuit 35 in the carrier terminal portion CT1 are normally deenergized. They are adapted to be energized from the power supply 5I through the made right-hand, parallel-connected switching contacts of either of the two switching relays 4I and 42 when that relay is operated. The relay 4I operates in response to the rectied output current of the demodulator 50 in the receiving circuit 35 when a received carrier wave of the frequency fn is applied to the input of the latter demodulator; and relay 42 operates in response to ringing current, say, of the frequency 20 cycles per second, transmitted from the central oice CO over the voice frequency line 39.

Similarly, the portion GT2 of the carrier terminal CT, associated with carrier channel 2 on the power line I, comprises a transmitting circuit 52 and a receiving circuit 53. The output of the transmitting circuit 52 and the input of the r ceiving circuit 53 are connected in parallel with each other and through the common coupler device 2 to the line I. 'I'he input of the transmitting circuit 52 and the output of the receiving circuit 53 are connected by means of the hybrid coil 54 and associated line balancing network 55 to the two-wire Voice frequency line 56 leading to the common central telephone switching ofi'ice CO for the subscriber stations associated with both channels I and 2 on power line I, and utilized also for connecting the telephone subscribers on carrier channel 2 of power line I to any of a number of telephone subscribers stations outside the carrier system in the associated telephone network. The line 56 leading to the central oflice CO is normally held open through the normally open parallel-connected, left-hand switching contacts of relays 58 and 59 in parallel. The operation of either one of these two relays to close its left-hand contacts will complete the connection of line 56 to the central oice.

The transmitting circuit 52 includes in order, v

between the hybrid coil 54 and the coupler device 2, reading from left to right, an audio ampliiier 60, a modulator 6I, similar to the modulator 44, with associated carrier oscillator 62 for supplying thereto carrier oscillations of the high frequency annesso* oillce'v CO, the closing ofv the line loop 39' may-'be' madeto operate aA lamp or other-calling devicel to call inthe-'central oiiiceA operator; The operi ated-relay 4I-through its made right-handcon"-` t tacts will alsoconnectv thepower supply 5f|-toN the carrier oscillator f l5^` and amplifiersA 43i and 46' of Ythe transmitting circuit 35- ot carrier Vterminal l portion CT1 causingV thesev apparatusfelementsto l be' energized; Carri-er oscillations of theA fre lectivelyl transmitting the speech-modulated'and' l0 quency f1 willthen betransmitted from the; os#- uninodulated carrier'waves of the frequency-f12 received-overthe-line I, thehigh frequencyarn-v pli'er andfthe4 demodulator 61'; similar' to the demodulatorV 50, for demodulating' the'v applied' cillatorl'ltomodulator IiIIu and the unmodulated' component of Afrequency f1 appearing in the out+ put-'of'that modulator will be transmitted through the high frequency amplifier 461, `the band filterv carrier-signal of frequency i12 to voice frequencies' 15= lllI andv theV coupler device 2 tothe power lineV I and for rectiiying the unmodulated` carrierv of that frequency. Y

The audio amplier 60,' the high frequencytransmittingl amplifier 63' and the-carrier osci-l'- over which it will be transmitted.

Ateapch Othe'subscribers stations A, B on.l

carrier'channel'l; the carrier waves of frequency' f1' received-overtime lineV I from the carri-er terlator 62 associated with the'transmitting;circuitv g0=ininal CT will be: transmittedu through the-cou,-

52 are-normallydeenergized They-'are adapted`L to 'beenergized fromthepow'ersupply'through the' parallel-connected; right-hand switching*- contacts of f either of. the ,two switching relays 58`v and* 59' 58`- operates: in response4 toA the rectified output current of" thedemodulat'ortl in` receiving circuit 53 'whena received carrier: wave" of` the frequencyV fi'z is applied to: the' input ofJ the` latter pler device. 2*, the receivng'bandlter Il; theV receivingamplier I2" andthe demodulator I3"; andthefrectied output ofthe latter will cause: the. operation of theassociated relay 2.2` tocause when-that relay is-operated; The? relay 5A the busy signal lamp 2iltofbe operated frombattery 2'I, thus indicatingnto` the subscribersat allv stations of channel, lion' the power line'I latter is in use on acall'.

The subscriber at station A' then talks into his that the demodulatorand relay'ftoperates in` response' auf transmitter, and'his; speech currents after'amto theringing-current; say of4 Zucycleperseconcl,

transmitted'b from the central office CO over the' voicel frequency line56.

Thecomplete' operationtofv the system ofthe plification in theI audioamplier Bare combined in modulator?" with the carrier oscillations. of frequency fn from oscillator' 8. They resulting modulation products after'arnpliicationV in the inventionwill nowbedescribedwithreference to 35A amplifier 9 will pass throughA the transmitting Figs; 1, Zand 3:

Call between.. power: Ime telephonefsuh'scriher and outside telephonesubscriberl band fllterlat thestation and the coupler de vice 2. tothe line I and` will be Vtransmitted over that lineto the carrier terminalCT:

At the carrier terminalCTthe speeclfi-rnodu When a telephone subscriber in the carrierfsys 40'13'ed Carrier Wave 0f 'the frequency fn willjpass.

through theeentrai cnice, co, he iiftshis hami-V setl 3 l 'oil the switchhook llof hisI telephone set, which, in the manner previously. described in connection with Fig, 3, `causes-the. carrier'oscil lator itat his station to be energizedftof-supply carrier oscillationsof:V the normal carrierA fre.- quency` fn fto the modulator I ,in :the :transmitting circuit 3Y at the station and the carrier oscillator wat his station to beenergized-to supply` car-A rier. oscillations; of thenormal carrier frequencyfito the demodulator- I3l inthe receivingcircuit. 4 at the station. The unmodu-latedcarrier comfponent.. ofthe frequency fu appearing. in the outv put of. modulatorn l.A willl be amplified in the `am-v plier 9=and the amplifiedV wave will. be trans.e

through the coupler device Z and' will be selec,- tively transinittedbyv the receiving circuit 3,6 of

the terminal portion CTi and, in the manner previously desoribed'for the unmoo'lulated, carrier. "wave of'frequency fn, passes through that circuit to the deinodulator 53'; The deinodulator 5U" demodulates the carrier signals toi/cioe frequencyv anda portion ofthe dernodulated signals is transmittedlthrough the hybrid coil 33" and the Voice frequency line loop 39'; held closed by the oper- Y atedlrelay 41to the central oflcerCO'and mhe..

heardI-by the central'ofll'ce operator. The sub,- scriber at station A" tells the centraloce operator t'owhich` subscriber station outside the carrier system he wishes to be connected; and' the operator in the usualmanner calls the desired subscriber." onl the'd associated telephone network and sets up a. talking connection throughtnecentral.' oficev CO so that the called and calling mitte@through theI assoeatedvband filter mand subscribersv can converse; with each other;

the. coupler. device 2 to the power linefl over which itwill be transmitted-to the carrier terminalCT..

The carrier wavesof frequency fu willlnass., through thecoupler device 2 at1 thecarrier terminalCT andlwill be selectedby the directional bandfllter 48 in -thereceiving..circuit 36. of. carrier terminal portionCTi. The selectedcarrier of'frequency. fn will be amplified, inamplifier 149Y the4 operation of. the relay 4|' connected tothe output of". the. latter.. The operatedV relayf4|v through its made. left-hand` contacts. will,V close, the.' lineloopljto the -central'oi'ce C O`.. By any suitable connection (not shown) in the central' 75` pliii'ed in amplifier d'6' and passed through the The Voice signals of the called" subscriber willV a and will te` impressed, by the hybrid @011131- on the transmitting circuit 35. In that circuit; the called'subscribersvoice signals will beanipliii'ed in theaudio amplifier I3l` and the amplified' signalsA modulated in modulator 44 withthe and Willbe rectified infdemodulator 50' causing 70l carrier: oscillationsV of` frequency f1 from carrieroscillatnr 45. previously energized in response to the. received unnrodulatecl carrier of frequency' The voiceemodulated' carrierA of frequency aficionesV 1l transmitting band filter 41 and coupler device 2 to the power line ,l and over that line to the calling station A. At the latter station, the voicemodulated carrier wave of frequency fi will pass through the coupler device 2 and will be selected by the band filter II in the receiving circuit d. The selected carrier signals will be amplied in amplifier I2 and then combined in demodulator I3 with the carrier oscillations of the normal frequency f1 supplied by the demodulator oscillator Ill, the associated frequency change key 32 being unoperated, to demodulate the voice signals which are heard 'by subscriber A in his telephone receiver I5.

At the end of the call, the subscriber at station A of carrier channel I will hang up his hand set on his hook switch I7 causing the opening of the switching contacts of the latter to disconnect the power supply from his modulator carrier oscillator 8 and his demodulator carrier oscillator Ifl thus deenergizing these oscillators. The deenergization cf the carrier oscillator 8 will stop transmission of the carrier wave of frequency fn over the line l. At the carrier terminal CT the relay di which had been held operated by carrier frequency fn will release to remove power from the carrier oscillator and the amplifiers in transmitting circuit 35 so as to stop the transmission of the carrier frequency f1 over the line, and also to open voice frequency line 38 to disconnect the central oflice from the branch CTi of the carrier terminal, thus returning the latter terminal to its normal unoperated condition shown and advising the central office operator to take down the connection.

The operating procedure in the case of a call from one of the subscribers stations X, Y associated with channel 2 on the power line I to a subscriber outside the carrier system to which the first subscriber is assigned, will be similar to that just described for a subscriber associated with channel l except that the normal transmitting carrier frequency Jiz for the former station will be used in transmission from the calling station to the carrier terminal CT, instead of the carrier frequency fn, and the normal receiving carrier frequency ,f2 of the former stations will be used for transmitting from the carrier terminal CT to the calling station, instead of the carrier frequency fi. The operation of the transmission and control equipment in the branch GT2 of the carrier terminal CT in connection with such a call over carrier channel 2 will be similar to that of the corresponding equipment in carrier branch CT1 previously described in connection with a call over carrier channel I.

A call to a subscriber on any carrier channel of the power line I may be initiated by the operator at the central office CO at the request of a subscriber outside the carrier system in the following manner: It will be assumed that an outside subscriber associated with the central of iice CO informs the central office operator that he desires to communicate with one of the subscribers associated with channel 2 on the power line I. The central ofce operator will then transmit ringing current, say of 20 cycles per second, over the two-wire line 56 to operate relay 59 connected across that line. The operated relay 59 through its made left-hand contacts will complete the connection of the line 56 to hybrid coil 54 of the carrier terminal branch CTZ allowing this ringing current to enter the transmitting circuit 52 of that branch. The operated relay 58 through its made right-hand contacts will cause the carrier oscillator 62 and the amplifiers 60 and 63 associated with transmitting circuit 52 to be energized from power supply 68. The ringing signals entering transmitting circuit 52, after ampliflcation in the audio amplifier 68, will be combined in the modulator 6I with the carrier oscillations of the frequency f2 supplied from the now energized carrier oscillator 62, The resulting ringing current-modulated carrier wave of the frequency f2 will be amplified in the high frequency amplifier 83 and transmitted out through the band lter 64 and the coupler device 2 to the line I over which it Will be transmitted to the carrier subscribers stations connected to that line. The portion of this modulated carrier of frequency f2 diverted from the power line I by the coupler device 2 at each of the subscribers stations X, Y in the group of stations associated with carrier channel 2 will be selected by the receiving band lter I I amplified by the amplifier I2 and will be combined in the demodulator I3 with the carrier of the frequency f2 supplied from the associated oscillator I4 to demodulate the ringing current which will cause the operation of the associated station signa1 (ringer) I9 and the busy lamp 28 at the station in the manner previously described for the similar subscriber stations associated with carrier channel I.

As the relay 59, connected across voice frequency line 55 Will remain operated to send out the ringing current-modulated carrier of the frequency f2 only during the times of application of the ringing current to that relay, the central 0fiice operator by interrupting the transmission of the ringing current at predetermined intervals in accordance with a particular code assigned to the station to be called may produce corresponding interruptions of the transmitted modulated carrier of frequency f2 and thus of the operation of the station signal device or ringer I9 at each of the stations X,Y

The subscriber at the called station, recognizing his coded ring, answers by removing the handset 3| from the hook switch II at his station. This will cause his modulator oscillator 8 and his demodulator oscillator I4 to be energized. The carrier oscillations of the frequency ,frz then will be supplied by oscillator 8 to the modulator 'I at the station, and the carrier component of the frequency i12 appearing in the output of that modulator will be amplified by amplifier 9, selectively transmitted by the band filter l'andwill pass through the coupler device 2 to the line I over which it will be transmitted to the carrier terminal CT. At the carrier terminal CT the received carrier wave of the frequency fizlwill pass through the coupler device 2 and will be selectively transmitted by the band filter 65 in the receiving circuit 53 in carrier terminal branch GT2. The selected carrier of frequency i12 after amplification in the amplifier 66 will be rectified in the demodulator 51 and will cause the operation of the associated relay 58. The operated relay 58 through its made left-hand contact will close the transmission line loop 56 to the central office 51, this loop having been reopened bythe release of the relay 59 with the cessation in the supply of ringing current from the central oiiice CO. The operated relay 58 through its made right-hand contacts will simultaneously connect the power supply 68 to the carrier oscillator B2 and the amplifiers 68 and 63 in the transmitting circuit 52, for energizing this equipment so that a carrier wave of the frequency f2 will be transmitted out over the line I, thus setting up the system-forV two-sway,` conversation` between the calling and called subscribers through fthe lineI,

the `branch CTzof the carrier terminal CT, the.V

linev 6and the central oilicefCO.

Thel conversation of the subscriber at the calledv station of channel 2li on the power lineI I will be carrier signals will be demoi'lulated` tovoiceA frequenciesv by: the demodulator 6 1 in there'ceiving y circuit 53- and will beitransmitted atvoice frequencies through the hybrid coil 54'; thev voice frequency line 56 and the central oii'ice CO-tofthe callingYV subscriber. The conversation of thecalling subscriber! will pass at` voice' frequencies through the central oiceCO and `over the` line tov-carrier terminalbranch C'Iz, and throughthe` hybrid coil 54-into the transmitting circuit 52J of that carrier terminal branch. These voicel signals will 'bei modulated inl modulator 6I in 'thatcircuit with the carrier of frequencyfzvfrom oscillator 62, and the voice-modulated carrier of that frequency` inamplified form will be transmitted-to the :'power line I fandlout over'y thatline to thecalled 'subscriber' in themannerl previously described.

CalZ'between two power line carrier. subscribers? stations' 'Ifo-make apa-rtyline orreverting call, say to` the*subscriber's` station Bonchannel I of the power' line I-,- the subscriber atlstationA ofV that f channeli'lrst determines bythe condition of hisoseill'torsiandf I4--to-shift the *tuning of these-A oscillators =froin frequency fu to ni12-and from-frequency-A fr to f2; respectively, the changed fre quenciesfcorrespondingto the-transmitting and' receivingfrequencies, respectively, employed on' carrier channel-2; and tochange'the connections at the station to connect the appropriate band lters I0 and II in the transmitting andreceiv'-l ingf'circuit, respectively, at` the station inplace offtheba-ndiilters I0 and II. If channel 2 at the time is also idle, the subscribers bus'ysignal lamp lll will rerriainL unlighted due to the absence of the carrier frequency f2 on the power line. SubscriberfA Athen liftshishandset 3l off the` switchhook `v IIgfwliich, by completing alockingecircuit` for r'elayf33', locksr his communica-tion set on theV alternate frequency assignment. The subscriber Almayithenrelease hislkey- 32.- The carrier frequency fizlappearing lin thel'output of modulator-1 will be transniittedlthrough the band the line I.

A portion of the carrier wavelof frequency fiz diverted from thedine Il at the carrier terminal filter I0 to Cthrouglilthe coupler device 2 willtbe-selectively` transit-litten by the bandll-ter 55 `in the receiving circuit53'iof branchCTaof the terminal. The

selected carrierof frequency fiz i will I be amplied.-

in ampl-ieri andrectiedin thefmodulator 6l to! cause1the-operation of'4 the associated relay 58. Tine: operated relay: 58 closest the line: loop 56r to the centrallofc'eCOrto call in thecentraloiiice operator and also `causes energizing current tobe supplied froml power supplyl 58# to the carrier' oscillator 52 `and the amplifiers 60 and. G3 associated With thef transmitting circuit 52 of carrier branch CT2so that a carrier wave` of the frequency f2 is transmitted out over that circuit to the line I over which it will be transmitted to the' subscribers stations. A portion of the carrier wave: ofzfrequency f2 will be diverted from the power line I through the coupler device 2 .at each of the subscribersstations X, Y associated with channel 2, will be selected by the receiving band filter I'I normally associated with the receiving circuit-4 :in each ofthese stations,- amplia ed in amplier l2 andrectied inV demodulator 3110 cause the operation of relay 22 to operatively energize the busy signal lamp at Z0, so as toindicate by the lighted conditions of the busy lamps to thesubscribers on channel 2 that the channelis busy in connection with a call.

The` calling subscriber at station A by talking into his transmitter 5 then modulates Vthe transinitted. carrier wave of frequency frz in modulator l-withhis voice signals, and the voice-modulated carrier wave of lthat frequency after amplication inthe amplifier. 9 will be transmitted through bandfilter lll' andcoupler device 2` to the line I over which-it will'be transmitted to the carrier terminalCT and.iwill.pass through the coupler device'` l:- at that terminal to the demodulator 6l inxthe receiving circuit 53 of carrier terminal branch GT2: in the manner previouslyk described for theunlmodulated'carrier of that frequency.

Thedemodulator el will dem-cdulate the receivedv centrall ofce operatorv then ringsstationB; asv

requested'bythewsubscriber at station A, in the manner-previously described in connection with a` call from an outside subscriber. The called subscriber'vB; on operation of the signal device (ringer) Ieajt his station to provide the'signal assigned to his station,` answers by lifting the handset 3.1 on" the switchhook il in his; ccmmunication set, causinghis. carrier oscillators t and Iii/tube energized to transmit carrier'oscilla tions of' the normalcarrier frequencies fn and:

f1' to the modulator l in his-transmitting circuit 3,and to the vdemodulator i3 in receiving circuit 1, respectively,

The unmodulated carrier component of fre'- quen'cy fn appearing in. the outputy of the' modulator 'l at' the called station B is transmitted through th'ebandV filter It normally associated with the transmitting circuit 3` at the station to the line I-and out over' the line to the carrier terminalCT. At that terminal, in a rmanner similari to. thatrdescribed previously in connection with a call to` an outside subscriber through the from the power line il at each of the stations A, B'. associated'. with channel I through: a coupler device 2 and is selectively transmitted-.by the bandflter l! normally associated with the receiving circuit 4 in each ofrthese stations. The selected carrier of frequency ,f1 in the output` oiV the' band lter lll after. amplicationin theA am- A portionof this. unmodulated carrier of frequency f1 is divertedv pliner I2, is rectified in the demodulator I3 to cause operation of the relay 22 associated with the output of the demodulator which in turn causes the busy signal lamp 2t to be energized in a manner previously described to indicate to the subscriber at the station that channel I of the power line is being used on a call.

The central office operator on the operation of his signaling device indicating that the subscriber at station B has answered, completes a talking connection througrnthe central' office CO between the voice frequency lines 56 and 39 so that the subscribers A and B on channel I can converse with each other.

The voice signals of the calling subscriber at station A normally assigned to channel I are transmitted as modulations of the alternate transmitting carrier frequency fm from the output of modulator I at that station out over the line I to the carrier terminal CT and through the receiving circuit 53 of carrier terminal branch GT2 to the demodulator 61 in that circuit; at voice frequencies from the output of that demodulator through hybrid coil 54 and voice frequency line 56 to the central office CO and from that cnice over voice frequency line 39 to the modulator 44 in the transmitting circuit 35 of the carrier terminal portion CTi; and from the output of the latter modulator as modulations of the carrier frequency f1 out over the line I to the demodulator I3 in the receiving circuit II of the called subscribers station B. As demodulated voice signals in the output of that demodulator are transmitted to the telephone receiver I and are heard by subscriber B.

The voice signals of the called subscriber B normally assigned to channel I, however, are transmitted as modulations of the normal transmitting carrier frequency fn of his station from the output of modulator 1 at station B out over line I to the carrier terminal CT and at that terminal to the demodulator 5!) in the receiving circuit 36 of carrier terminal branch CT1; at voice frequencies from the output of the demodulator 50 through the hybrid coil 31 and voice frequency 39 the central ofce CO, and from the central oflice through the voice frequency line 56 and hybrid coil 54 to the modulator 6I in the transmitting branch 52 of the carrier terminal branch GT2; and from the output of that moduy lator as modulations of the transmitted carrier frequency f2, which is the alternate receiving carrier frequency of the calling subscriber A, to the line I and over that line to the demodulator I3 in the receiving circuit 4 of the calling subscribers station A. The demodulated signals of subcriber B appearing in the output of demodulator I3 at subscriber station A passes to the associated telephone receiver I 5 and are heard by subscriber A,

When the call between subscriber stations A and B is completed, both subscribers hang up their desk sets 3| on switch-hook I'I of their communication sets. This automatically deenergizes the carrier oscillators 3 and I4 of both sets, stopping the transmission of the carriers fn and f1.2 over the line I so as to signal the central oice operator to pull down the previous connection; reverts the tuning oi the oscillators at the calling station A to the frequencies, fn and ,f1 of channel I to which that station is normally assigned, and deenergizes the changeeover relay circuits R1 and Rz at that station to switch the filter connections back to normal.

N o mention has been made previously of the loss pad 68 inserted in the connection between the coupler device 2 and the band filters I0, IU', II and II at subscriber station A. This pad is provided to insert a fixed amount of loss between subscriber station A and the carrier terminal CT, because of the closer proximity of that station to the carrier terminal than the other subscriber stations, so as to maintain substantially the same amount of net loss between the carrier terminal and each subscribers set on the system. Loss pads of suitable values may be inserted in the connections to each of the other subscribers stations also, if required to accomplish the above purpose.

Some of the advantages of the carrier arrange'- ments of the invention are outlined below:

1. No modifications in the switchboard at the central office are required. All switchboards, Whether dial or magneto, include provisions whereby closure of the loop 'in accordance with switchhook (or dial) operations will perform all the functions required.

2. No sequence operations in the carrier terminal are required to diiferentiate between a straight (outside) call and a party line call, thus resulting in a saving 1in equipment and simplification in operating procedure.

3. TheY subscribers set yat each subscribers station operates only in conjunction with a carrier terminal so that loss pads can be provided with sets close to the carrier terminal to permit a fixed net loss between the carrier terminal and each subscribers set on a carrier channel. Thus, it will not ordinarily be necessary to provide AVC (automatic volume control) arrangements either in the subscriber sets or carrier terminals. However, it is within the scope of the invention to provide such AVC arrangements in connection with the amplier at each of the subscribers sets or carrier terminals if desired.

4. It permits a large reduction in selectivity requirements because:

(a) Due to the loss pad arrangement outlined above, there will be no large level difference at any point on a line between the outputs of the various subscriber sets on several channels associated with the line. Thus, crosstalk requirements between channels going in the same direction on the line can Ibe minimized to the point where equal level crosstalk only involved;

(b) For all channels on the line, the channels going in one direction, that is, from the carrier terminal to the subscriber, can be grouped in one frequency range and the channels going in the opposite direction, from subscribers to the carrier terminal, can be placed in another frequency range, thus affording larger frequency separation for conditionswhere opposite directional transmission is involved. l

Various modications of the carrier arrangements illustrated and described which are within the spirit and scope of the invention will occur to persons `skilled in the artl What is claimed is:

1. In a carrier communication system, awave transmission medium, a central station including at least two carrier terminals connected to said medium, operating on different carrier frequencies to transmit communication signals to and receive communication signals from said medium, a plurality of carrier subscriber stations connected to said medium, each of one group of said subscriber stations normally operating to transmit communication signals to and to receive communication. signals` fromVv saidlmedium on the: receiving carrier frequency" andthe transe mittingf carrier frequency, respectively,A of one vof said 4carrier terminals, each ofrasecond'group of'` said. subscriberV stations normallyoperating; to

transmit communication signals toandto receivecommunication. signals fromsaidmedium on ythe. receiving carrier frequency and the transmitting carrier-frequency, respectively, of another ofsaidl carrier terminalsandrneans toA enable communi` cation without appreciabledistortion between'any two subscriber stations in either group. comprising means at each subscriber station in` each` group for changing the station transmitting and receiving carrier frequencies to those ofsaid other group and means for interconnecting the carrier terminals of th'e two groups so that the signalsreceived by each carrier terminal over said medir umon thev receiving carrier frequency offthat1 said. individual carrier terminal: stations, each ofA saidcarrier channels normally utilizing carrier waves of different frequencies, different for each channel, for the'communication signals transmitted in opposite directions between its carrier terminal station. and. itsxsubscrlber stations and means to enable communication without appreciable signal distortion between any two of said subscriber stations associated with the same carrier channel comprising means at one of said two stations under control of the subscrib er thereat for changing its transmitting -andreceiving carrier frequencies to those normally utilized by the subscriber stations associated with another carrier channel and means'under-control of said central oiiice for connecting the carrier terminal station of said other channel to the carrier terminal station of said same carrier channel so as to supply the communication signals received by each carrier terminal station from said medium to the other for transmission thereby on its normal transmitting carrier frequency over said medium.

3. In a carrier wave communication system, a, transmission medium, a plurality of groups of subscriber carrier communication stations and a corresponding number of two-way carrier terminals connected to said medium, one group of said subscriber stations and one of said carrier terminals associated with one carrier channel on said medium normally utilizing one carrier frequency for transmission in one direction between each subscriber station in that group and said one carrier terminal and a second carrier frequency for transmission in the opposite direction therebetween, a second group of said subscriber stations and a second of said carrier terminals associated with a second carrier channel on said medium normally utilizing a third carrier frequency for transmission in One direction between each subscriber station in the group and said second carrier terminal and a fourth carrier frequency for transmission in the opposite direction therebetween, a common central switching ofiice associated with said medium separately through thel carrier. terminal station for eachV carrier ch'annel-and means toenable two-way communi cation' without appreciable signal distortion be tween `any two subscriber stationsnormally ass'ociated with the samecarrier channel compris,-

ing means-under control `of the subscriber at the one of said two stations calling the otherA for changing the carrier transmitting andreceiving frequenciesiof said one station only to those normallyutilized:` by the subscriber stations of an other channel, and means under controlu of said central switching oiiice for setting up aconnection betweenthe carrier terminal stations for said same and said other channel such that the communication signals received by each from said medium are retransmitted by the other on its normalA carrier transmitting frequency over said medium.

le Ina carrier communicationsystem, a transmissionmeclium, a central station including ati leasttwo `two-way carrier terminals, a plurality of two-way carrier substations connected'to said medium, each of said carrier terminals operating onA respectivelydierent carrier frequencies for transmission to and from said medium,.on`e, group of said carrier substations normally operating on' the carrier frequencies of one of said carrier terminalsfor transmission to and from said medium,V another group of said carrier substations normally operating on the carrier frequencies of another.` of said carrierv terminals for transmission to and from said'mediurn and means for changing the operating frequencies of any substationin one of. said groups to the normal operat- Y ing` frequenciesof the substations in anotherof said groups to enable communication through said central station between substations in the samegroup.,

5.1Infa carrier-,communication system', a transmission medium, a plurality of groups of sub'- scriberfs two-Way carrier communicationsets and a corresponding number of two-way carrier terminal sets connected, to said medium, a common central switching office for said subscriber sets adapted for connection to said medium through each of'said carrier terminal setsVeacli set. of one group of'said subscriber sets being normally conditioned under control of the subscriber to transmit to said medium on a carrier of one frequency and to` receive therefrom on a carrier of a second frequency, each set of a second group of said subscriber sets being normally conditioned under control of the subscriber to transmit to said medium on a carrier of a third frequency and to receive therefrom on a carrier wave of a fourth frequency, one of said carrier terminal sets being conditioned to receive from said medium on a carrier of said one frequency, and t0 transmit thereto, under control of said central olice or a received carrier of said one frequency, on a carrier of said second frequency, a second of said carrier terminal sets being conditioned to receive from said medium on a carrier Wave of said third frequency and to transmit thereto, under control of said central oiice or a received carrier of said third frequency, on a carrier wave of said fourth frequency, and additional means in each' subscriber set in said one and said second group under control of the subscriber to change the transmitting and receiving carrier frequencies of the set to the normal transmitting and receiving carrier frequencies of the subscriber sets in the other group.

6. The system of claim 5, in which said common central ofiice is normally disconnected from said medium and each of said carrier terminal sets includes switching means under control of said common central switching office or of a received carrier of the frequency on which it is conditioned to receive, to connect said central o'ice to said medium through that carrier terminal set.

7. The system of claim 5, in which each of said carrier terminal sets includes means for demodulating the communication signals from the carrier of the frequency which it is conditioned to receive, incoming over said medium, for transmission to said central oce and means for modulating signals from said central o'ce on the carrier transmitted by the set to said medium.

8. In combination in a carrier communication system, a wave transmission medium, a plurality of two-way carrier communication channels on said medium, a plurality of groups of subscriber two-way carrier communication stations and a corresponding number of two-way carrier terminals connected to said medium, a common central switching oiiice connected to said medium through each of said carrier terminals, one group of said subscriber stations and one of said carrier terminal stations being associated with one of said carrier channels, another group of said subscriber stations and another of said carrier terminals being associated with another of said carrier channels, and means to enable two-way communication between two subscribers stations in the same group through said central oiiice comprising means at each subscriber station in said one and said other group under control of the local subscriber for shifting its transmitting and receiving carrier frequencies to those of the subscriber stations in a different group.

9. In combination in a carrier communication system, a wave transmission medium, a plurality of subscriber two-way carrier communication sets and a plurality of two-way carrier terminal sets connected to said medium, a common central switching office for said subscriber sets connected to said medium through each of said carrier terminal sets, each of one group of said subscriber sets being normally adapted to transmit communication signals to said medium on a carrier wave of one frequency and to receive communication signals therefrom on a carrier wave of a second frequency, each of another group of said subscriber sets being normally adapted to transmit communication signals to said medium on a carrier wave of a third frequency and to receive communication signals therefrom on a carrier wave of a fourth frequency, one of said carrier terminal sets being adapted to selectively receive carrier waves of said one frequency incoming over said medium, to demodulate the communication signals therefrom for transmission to said central oiiice and to transmit communication signals received from said central ofce to said medium on a carrier Wave of said second frequency, a second one of said carrier terminal sets being adapted to selectively receive carrier waves of said third frequency incoming over said medium, to demodulate the communication signals therefrom for transmission to said central ofce and to transmit communication signals received from said central office to said medium on a carrier wave of said fourth frequency and each subscriber set in said one and said second group also including means under control of the local subscriber for changing the transmitting and receiving carrier frequencies to those normally used by the subscriber sets in the other group to enable two-way communication through said central office and said one and said second carrier terminal sets between subscriber sets in the same group.

MILTON L. ALMQUIST.

REFERENCE S CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,383,908 Bowers Aug. 28, 1945 OTHER REFERENCES War Dept. Publication TM 'l1-600, Nov. 1943. 

